A Light-Emitting Diode (LED), as used herein, is a semiconductor light source including a semiconductor diode and optionally photoluminescence material, also referred to herein as phosphor, for generating a light at a specified wavelength or a range of wavelengths. LEDs are traditionally used for indicator lamps, and are increasingly used for displays and general illumination. An LED emits light when a voltage is applied across a p-n junction formed by oppositely doping semiconductor compound layers. Different wavelengths of light can be generated using different materials by varying the bandgap of the semiconductor layers and by fabricating an active layer within the p-n junction. Additional phosphor material changes the properties of light generated by the LED.
In LED lamps, multiple LEDs are often used in a circuit to generate the light output by the lamp. A white light LED usually generates a polychromatic light through the application of one or more phosphors. The phosphors shift blue light or other shorter wavelength light to a longer wavelength through a phenomenon called a Stokes shift. The perception of white may be evoked by generating mixtures of wavelengths that stimulate all three types of color sensitive cone cells (red, green, and blue) in the human eye in nearly equal amounts and with high brightness compared to the surroundings in a process called additive mixing.
LED lights can last longer and use less electricity than traditional bulbs and thus their use is becoming more widespread. However, the white point of the light can move as the different LEDs and phosphor age at different rates. User preferences for different color temperatures (warmer yellow versus cooler blue) of the white light also create a market for user adjustable color temperatures. Cost-effective and user-friendly methods to adjust color temperature are sought.